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J. A. BASSHAM, M. KIRK 



2.0 respectively). The resulting saturation curves are shown in Fig. 10. In the same 

 figure, Curve R is the saturation curve for ribulose diphosphate, obtained by dividing 

 the experimentally determined i*C labeling of RuDP by its steady state concentration, 

 which was 0.36 ^moles/ml algae. 



If the carboxylation of RuDP were to lead to the formation of two molecules 

 of PGA (reaction D), then all of the carbon atoms of RuDP must give rise to the 

 "residual" carbon atoms of PGA. The degree of saturation of these residual carbon 

 atoms at no time could exceed the degree of saturation of the carbon atoms of RuDP. 

 Since the calculated values for these residual atoms, (PGA-0.5 B)/2.5, do exceed 

 those of RuDP at all times after 12 sec, reaction D does not appear to be correct. 

 The curve for reaction L does not exceed the saturation of RuDP until about i min. 

 In this case, the residual carbon atoms of PGA are derived only from carbon atoms 2 

 and 3 of RuDP, and thus may exceed the saturation of the average of carbon atoms 



Time in Seconds 



"T5 'X 30 40 50 eo' 

 Time in Seconds 



ao 90 100 



Fig. 9. Degree of saturation (vs. time of photo- 

 synthesis with "CO2) of "CO2 pool" and of 

 PGA carboxyl derived immediately from "COj 

 according to two proposed carboxylation re- 

 actions. Curve A is for "COj pool", curve B is 

 for PGA carboxyl derived immediately from 

 ^^COj according to reaction D, curve C is for 

 PGA carboxvl according to reaction L. 



Fig. 10. Degree of saturation of ribulose di- 

 phosphate (R) vs. time of photosynthesis with 

 "CO2 compared with degrees of saturation of 

 residual carbon atoms of PGA according to two 

 proposed carboxylation reactions. 



I, 2, 3, 4, and 5 of RuDP. In fact, this is not surprising, since earher degradation 

 studies on RuDP' showed that, during i*C incorporation in photosynthesis, carbon 

 atom 3 is first labeled, followed by carbon atoms i and 2, followed finally by carbon 

 atoms 4 and 5. The saturation curve for the residual PGA carbon atoms according 

 to reaction L is thus about as would be expected. 



Note that after 30 sec the carboxyl carbon of PGA would be saturated and the 

 same conclusion could be reached by looking only at the curves from 30 to 90 sec, 

 which are not dependent on the foregoing calculations of CO^ pool and PGA carboxyl 

 saturation. At these longer times it is sufficient to plot simply the curves for (PGA-0.5)/ 

 2.5, (PGA-i.o)/2.o, and RuDP/0.32 all as a function of time. 



We conclude, therefore, that the labeling curves for PGA and RuDP in this 

 experiment can best be interpreted as resulting from the occurrence of reaction L. 

 That is, the in vivo carboxylation reaction of the carbon reduction cycle during 



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